Life support for patients with compromised respiratory function is typically maintained by the periodic introduction and extraction of a respiratory gas by mechanical means. The mechanical means, a ventilator, periodically introduces, under positive pressure, a respiratory gas into a tube inserted into the trachea of the patient. The portion of a mechanical ventilatory cycle during which a respiratory gas is introduced under positive pressure causes the gas to move into and inflate the lungs. Following the portion of the ventilatory cycle during which the respiratory gas is introduced under positive pressure, the ventilator reduces the gas pressure and the lungs deflate, causing the respiratory gas to be passively exhaled. The ventilatory cycle provides the movement of oxygen into the lungs and the removal of carbon dioxide from the lungs necessary to keep the patient alive. Following the portion of the ventilatory cycle during which the respiratory gas is passively exhaled, the portion of the ventilatory cycle during which the respiratory gas is introduced begins again and the cycle is thus repeated.
Although ventilation technology has been known for more than one hundred years, recent advances in control technology using microprocessors have permitted physicians to control a wide range of parameters, such as end exhalation pressure values, tidal and total volume and respiratory gas introduction/extraction times, in order to optimize the patient's respiratory function. However, it has relatively recently been determined that attempts to optimize ventilatory function may result in an unwanted and potentially dangerous interference with cardiac function.
The interference with cardiac function is a result of the heart and the lungs being commonly located within a relatively rigid chest cavity. As positive pressure is applied to the lungs and the lungs expand, they occupy a larger volume within the chest cavity and increase the pressure around the heart and compress the veins returning blood to the heart. This pressure, if applied during the diastolic or filling phase of the cardiac cycle, prevents the heart from filling and ejecting adequately and has been shown to decrease cardiac output by as much as 50%.
Investigators have found however, that if positive pressure is used to inflate the lungs predominantly during the systolic or contraction phase of the cardiac cycle, the external pressure exerted by the lungs on the heart aids in the emptying of the heart and hence improves cardiac output. To use this technique, the investigators have typically utilized ventilatory frequencies which are at, or at a subharmonic of, the heart frequency. The use of a ventilatory frequency which is at the cardiac heart rate frequency results in a ventilatory rate (75-150 breaths per minute) which is much higher than the normal ventilatory rate (10-75 breaths per minute). Such a high frequency of ventilation requires a subnormal tidal volume to be used which, in turn, hinders the maintenance of normal physiological oxygen and carbon dioxide concentrations. In addition, the high frequency rate of ventilation also results in a higher average airway pressure, which can cause lung injury and, in itself, can interfere with the normal mechanism of cardiac filling and ejection.
The present invention relates to a method and apparatus for timing the respiratory gas introduction and extraction portions of the ventilatory cycle so as to be synchronous with portions of the cardiac cycle.